1. Field of the Invention
The present invention relates to a novel production method for optic, optoelectric, and microelectronic structures and devices. More particularly, the present invention relates to a method of patterning photosensitive polymeric dielectric compositions using electron beam lithography.
2. Description of the Prior Art
Processes for accurately and routinely patterning sub-micron features in a variety of materials are critical for the future of a wide range of devices and structures for applications in areas ranging from biological substrates to electronic, optoelectronic and molecular devices to Micro Electro-Mechanical Systems (MEMS) structures. Depending on the specific application, the material being patterned may be just as critical, since its electrical, thermal, optical and physical characteristics may ultimately determine its viability.
In addition, nano-scale devices are the topic of intense research and development for a variety of electronic, optoelectronic and biological applications. In the optical/optoelectronic area, structures such as 2 and 3 dimensional artificial photonic crystals can exhibit a wide range of useful and unique characteristics, which can be used to develop many important devices for commercial applications. Nano-scale Electro Mechanical Systems (NEMS) are another important example of 3 dimensional structures, which have great commercial potential in areas ranging from sensors to optoelectronics to biological applications. In the electronics arena, nano-scale devices offer the potential for a significant increase in the density of fabricated devices and related circuits, improved operating characteristics, and exhibiting entirely new behavior. Specific examples include molecular devices, such as molecular transistors, which may be fabricated in densities far exceeding those attainable via currently used processes, such as those used to fabricate DRAM memory elements.
Another very important matter is the substrate on which molecular devices, or other nano-scale devices, can be fabricated. Although silicon is the substrate material currently used to manufacture most electrical semiconductor devices today, emerging organic-based device technologies may allow nano-scale devices to be fabricated on other materials, including polymers. These technologies may have a significant impact on, for example, the biological, pharmaceutical and display industries.
In the electronics arena, given the current focus on substantially reducing the size of devices, as well as developing device technologies which are compatible with a variety of materials and applications, a robust multi-level interconnect process may ultimately determine the level of integration achievable for electronic circuit applications. Interconnects not only connect devices to the ‘outside world’ but also serve as the electronic pathways for sending signals from one device to a neighboring device or to many devices located physically on the other side of an integrated circuit. If new technologies allow devices to be reduced in size to <100 nm, then the interconnect system may ultimately limit how small a circuit using these devices can be made.
Of course, a multi-level interconnect system is just one example of a general 3 dimensional metallic structure. As mentioned above, other devices, such as 2 dimensional gratings, 2 and 3 dimensional artificial photonic crystals and NEMS may have many important commercial applications, but their fabrication process remains challenging and complex.
One excellent material currently being used to fabricate electrical interconnects (but could be used to fabricate a variety of 3 dimensional and other structures) is a bisbenzocyclobutene derived composition available commercially as CYCLOTENE® from The Dow Chemical Company. CYCLOTENE® resins are high-purity polymer materials that have been developed specifically for microelectronics applications. The resins are derived from B-staged bisbenzocyclobutene (BCB) monomers and are formulated as high-solids, low-viscosity solutions. This material has been used in photolithography applications and is exceptional electrically, as it exhibits a low dielectric constant (approximately 2.6, much better than Silicon Dioxide, which has a dielectric constant of 3.9), making it ideal for high frequency electronic circuit applications. It also exhibits a very high resistivity (1×1019 ohm-cm) and a low dissipation factor (0.0008 at 1 kHz-1 MHz). Bisbenzocyclobutene also has other desirable physical and optical characteristics. Excellent planarization characteristics can be obtained with typical deposition processes (greater than 80% planarization for height variations greater than 1 μm easily achievable). In addition, the material is transparent in the infrared region, which may enhance its usefulness to biological and sensor applications.
In addition to the desirable material characteristics discussed above, bisbenzocyclobutene also has other desirable characteristics making it particularly attractive for certain device fabrication processes. This material is resistant to many wet and plasma based etch chemistries, making it suitable for masking and other process steps.
Currently, there are only two known approaches for patterning bisbenzocyclobutene. One approach is to deposit the material and then use another material which can be patterned (typically a polymer known as a resist, which is used extensively in the semiconductor processing industry) which serves as a mask for etching the bisbenzocyclobutene. This process may be quite difficult depending upon several factors including thickness of the bisbenzocyclobutene, feature size of the mask and the available etch processes and resists.
The most direct approach, however, is to use a photosensitive version of bisbenzocyclobutene. In this case, the patterning of the material is accomplished using a process very similar to those used for typical resists implemented in the semiconductor processing industry. In this case, ultraviolet light is used to modify the physical and/or chemical properties of the material to make the material more or less susceptible to removal via another chemical referred to as a developer. This process is typically referred to as optical lithography. In particular, exposure to ultraviolet light changes a positive resist from polymerized to unpolymerized and a negative resist from unpolymerized to polymerized. The polymerization process causes the resist to become a cross-linked material, which cannot be removed using a developer. In a negative working resist composition, the light struck areas form the image areas of the resist after removal of the unexposed areas with a developer. In a positive working resist the exposed areas are the non-image areas. The photosensitive version of bisbenzocyclobutene behaves like a negative resist since exposure to ultraviolet light causes the material to convert from an unpolymerized to a polymerized state.
One critical issue with this (and any) approach to patterning materials is the ultimate resolution achievable for a given process, i.e., how small a feature of the material can be patterned. In the case of optical lithography, the ultimate resolution is typically related to the wavelength of the ultraviolet light used for exposure. The smaller the wavelength of light used, the better the achievable resolution. Thus optical lithography limits the attainable feature size since it relies on ultraviolet light to pattern the material.
In order to fabricate structures with smaller feature sizes, other lithographic processes have been developed. In particular, processes using electron beams in place of ultraviolet light have been developed since electron beams can be generated and controlled with a resolution of approximately 2-10 nm, far exceeding the resolution attainable using conventional optical lithography. Of course, to exploit the resolution achievable via electron beams, resists sensitive to electrons instead of light have been developed. Currently available resists exhibit good resolution, but have several drawbacks. They typically have poor resistance to plasma based etching and cannot typically be used as a permanent part of a device structure, i.e., they are used to pattern a material but then are removed. There are many applications, such as the fabrication of multi-layer nano-scale interconnect architectures for electrical, optical and, in particular, molecular devices, which would substantially benefit from a process for patterning more suitable materials on the nanoscale, which exhibited superior material or process-related characteristics.
In addition, the dose required to properly expose an e-beam resist can be relatively significant, on the order of 300 to 900 micro Coulombs per square centimeter. An electron beam resist which required a lower dose for exposure could substantially reduce the time required to pattern a sample, lowering the cost of e-beam lithography and increasing the number of wafers patterned in a given period of time. It would be highly advantageous to remedy the foregoing and other deficiencies discussed above.
A further object of the present invention is to provide a method for fabricating multi-level nano-scale interconnect systems.